Automatic degaussing device

ABSTRACT

Degaussing apparatus for cathode ray tubes such as television picture tubes and the like utilizing positive temperature coefficient thermisters to control the current through a degaussing coil and means for heating the thermister to reduce the residual degaussing current to a relatively low value.

United States Patent Ihaya et al.

[ Oct. 29, 1974 AUTOMATIC DEGAUSSING DEVICE Inventors: Osamu lhaya, Kameoka; Munehiro Ito, Kyoto; Tetsuo Hayashi, Takatsuki; Shoichi Fujimoto, Kameoka, all of Japan Assignee: Nichicon Capacitor Ltd., Kyoto,

Japan Filed: Mar. 13, 1973 Appl. No.: 340,885

Related [1.8. Application Data Division of Ser. No. 112,471, Feb. 3, I971, abancloned, which is a division of Ser. No. 773,427, Nov. 5, I968, abandoned.

US. Cl. 338/23 Int. Cl l-I0lc 7/04 Field of Search 338/23; 323/68 [56] References Cited UNlTED STATES PATENTS 2,926,299 2/l960 Rogoff 338/23 X 2,976,505 3/l96l lchikawa 338/22 R 3,3l6,765 5/!967 Trolander et al. 338/23 X Primary Examiner-C. L. Albritton [57] ABSTRACT 1 Claim, 10 Drawing Figures PATENTEUnm 29 m4 SHEEIIIJFZ T a TEL-1-2.-

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- Taupe/Wren? AUTOMATIC DEGAUSSING DEVICE This is a division of application Ser. No. l l2,47l filed Feb. 3, I971, now abandoned, which application is a division of application Ser. No. 773,427 filed Nov. 5, I968, now abandoned.

This invention relates to degaussing devices for cathode ray tubes such as television receiving tubes and the like and more specifically concerns an automatic degaussing device having a very low residual current.

Known degaussing devices for cathode ray tubes have not been found to be satisfactory because the residual current in such devices is relatively high, and this current adversely affects the image produced on the tube. This invention overcomes the foregoing disadvantage and produces a residual current of a very small magnitude having little if any adverse affect on the image produced by the tube.

Another object of the invention resides in a novel and improved current control device for use in degaussing cathode ray tubes.

The above and other objects of the invention will become more apparent from the following description and accompanying drawings forming part of this application.

In the drawings:

FIG. I is an electrical circuit showing a conventional automatic degaussing device;

FIG. 2 is a cross-sectional view of a conventional thermister used in connection with the circuit shown in FIG. 1;

FIG. 3 is an electrical circuit illustrating an automatic degaussing device in accordance with the invention;

FIG. 4 is a cross-sectional view of one form-of thermister for use with the circuit shown in FIG. 3',

FIGS. 50 and 5b are side and cross-sectional views, respectively, of a modified thermister in accordance with the invention;

FIGS. 60 and 6b are side and cross-sectional views, respectively, of still another embodiment of a thermister in accordance with the invention;

FIG. 7 illustrates a characteristic resistancetemperature curve of a semi-conductor material of the barium titanate group having a positive temperature coefficient and utilized in thermisters; and

FIG. 8 illustrates in dotted outline the current characteristic of prior known devices for automatic degaussing and in full lines the improved current characteristic of the automatic degaussing device in accordance with the invention.

The conventional degaussing device illustrated in FIGS. I and 2 comprises a thermister generally denoted by the numeral 2 having positive temperature characteristic and connected in series with a degaussing coil 3. The coil 3 is normally disposed either in the vicinity of the shadow mask of a television tube or in the vicinity of the deflection coil usually disposed on the neck portion of the tube. The coil is energized by a suitable source of current 1 which causes an initially large degaussing current to flow therethrough. This current also functions to heat the thermister 2 thereby increasing its resistance and gradually reducing the current. The current characteristic of the structure shown in FIG. I is represented by the dotted curve (b) of FIG. 8, and it will be observed that the current stabilizes at a relatively large value.

The thermister used in connection with the structure shown in FIG. I is illustrated in FIG. 2 and embodies a body of material 4 having a positive temperature characteristic, conductive coatings 5 and 6 on each side thereof, and terminals 7 and 8 secured to the conductive coatings. The entire structure is then encased in a suitable protective nonconductive material 9.

The improved automatic degaussing device in accordance with the invention is illustrated in FIGS. 3 and 4. The thermister control 12 embodies two individual thermister portions 10 and 11 with the thermister portion 10 being connected in series with the degaussing coil 3 and energized by a source of alternating current I. The second thermister portion I1 is connected directly across the source of alternating current so that a current I, will flow through the coil 3, and a second current I will flow through the thermister portion 11.

The thermister 12 as illustrated in FIG. 4 includes two bodies of a semi-conductor material of the barium titanate group having a positive resistance temperature characteristic. These bodies are bonded one to the other by a suitable conductive material 16 and the outer surface of the bodies 13 and 14 have conductive layers 15 and 17. A terminal 18 is secured to the conductive layer 15, a terminal 19 is secured to the common conductive layer 16, and the terminal 20 is connected to the conductive layer 17. The entire structure is then enclosed within an insulating layer 21. The terminal 19 of the structure shown in FIG. 4 is connected to a source of alternating current 1 while the terminal 20 is connected to the coil 3. The other side of the coil 3 together with the terminal 18 is connected to the other side of the alternating current source. With this arrangement and by selecting appropriate resistance temperature characteristics of the semi-conductors l3 and 14, a current wave form such as that shown in full lines at (a) in FIG. 8 is attained. This occurs by reason of the added heating effect produced by the thermister portion II which greatly increases the heat developed in the thermister portion 10 and thereby materially reduces the residual current to a very small value within the desired time.

FIGS. 5a and 5b illustrate an alternate embodiment of a thermister in accordance with the invention. It will be observed that a single body of semi-conductor material 22 having positive temperature coefficient is provided with a conductive layer on one side thereof to which a terminal 26 is attached. The terminal 26 corresponds to the terminal 19 of FIG. 4. The other surface of the body 22 has a small U-shaped layer 23 of conductive material and a second partially surrounding layer 24 also of a conductive material but insulated from the layer 23. A terminal 27 is connected to the conductive layer 23, and this terminal corresponds to the terminal 20 of FIG. 4. A third terminal 28 is connected to the conductive layer 24 and corresponds to the terminal 18 of FIG. 4. With this arrangement an added heating effect is attained by reason of the improved thermocoupling between the two thermister sections and manufacturing costs are materially reduced. This form of the invention would also include a suitable coating of insulating material such as the coating 21 illustrated in FIG. 4.

A still further embodiment of the invention is illustrated in FIGS. and 6b and corresponding elements of this embodiment of the invention and that illustrated in FIGS. 50 and 5b are denoted by like primed numerals. The embodiment of of FIGS. 6a and 6b differs from the embodiment of FIGS. 50 and 5b in that the conductive layers 23' and 24' are concentric so that the heat generated as a result of current flowing between the conductive layer 24' and the conductive layer 25' will have a more pronounced effect on the central portion of the semi-conductor material 22 and thereby produce a more rapid reduction of current flowing through the coil 3.

Referring again to FIG. 3, when AC voltage from the power source is applied to the thermister 12, an AC current flows through the coil 3, which current is limited by reason of the IR drop in the thermister 10. Since the thermister has a positive temperature characteristic, the heat generated will be a function of the resistance R, times the square of the current l,. The heat thereby generated will increase the resistance of the thermister 10 until a thermobalance point is attained.

point if the thermister 11 was not utilized. Furthermore, the balance point will be achieved within about 30 seconds after the application of the AC voltage and the current in the degaussing coil will be as small as 1/500 of the residual current obtained with prior art devices. As a result, degaussing can be effected rapidly and at the same time an improved picture is produced on the cathode ray tube.

While only certain embodiments of the invention have been illustrated and described, it is apparent that alterations, modifications and changes may be made without departing from the true scope and spirit thereof as defined by the appended claims.

What is claimed is:

l. A thermister having positive temperature characteristics comprising at least one piece of resistive material having a positive temperature characteristic and a plurality of electrodes secured to said material to provide a current control portion and a heat generating portion, said resistive material having a pair of opposing surfaces witha layer of conductive material on one surface and at least two conductive layers on the other surface, the last said layers being spaced one from the other, and said electrodes being secured to said layers, one of said two conductive layers on said other surface at least partially surrounding the other of the last said layers.

t a: a a: 

1. A thermister having positive temperature characteristics comprising at least one piece of resistive material having a positive temperature characteristic and a plurality of electrodes secured to said material to provide a current control portion and a heat generating portion, said resistive material having a pair of opposing surfaces with a layer of conductive material on one surface and at least two conductive layers on the other surface, the last said layers being spaced one from the other, and said electrodes being secured to said layers, one of said two conductive layers on said other surface at least partially surrounding the other of the last said layers. 